One of the most striking features of the aging process is the associated decline in physical capabilities, including the slowing of movements, a decrease in strength, and a loss of fine motor coordination. Although the entire motor system seems to be involved in these age-related changes, more attention has been directed toward muscle architecture than to the properties of motor units. A thorough understanding of changes in motoneuron function with aging is critical. For example, as we age the number of motoneurons innerving a muscle decreases while the average size of each surviving motor unit increases. These changes undoubtedly influence the way in which the nervous system varies the force that a muscle exerts. The purpose of this project is to determine if the known age-related changes in motor unit number are associated with specific alterations in motor unit behavior during acute exercise, and whether chronic exercise (training) has a beneficial effect on these behavioral changes. It is hypothesized that the pattern of motor unit activation during simple exercises is different in older, compared to younger, subjects but that these differences can be lessened with training. Although the ultimate goal is to correlate changes in motor unit behavior with performance capabilities, this proposal consists of three specific aims that address the intermediate step of characterizing the effects of age on motor unit behavior. he dependent variable will be motor unit activity, as characterized by the forces at which the units are activated and inactivated and the pattern of action potential discharge. The experiments are designed to examine the effect of age on these properties as exhibited during a simple isometric exercise. In order to explore the functional implications of any age-related effects, motor unit activity during this exercise will be examined under three conditions:(1) tests will be conducted on a small hand muscle (first dorsal interosseus) and on an arm muscle (biceps brachii) to see if motor unit behavior, as it does in young subjects, varies across muscles:(2) since muscle fatigue is known to alter motor unit behavior during subsequent exercise in young subjects: we propose to contrast the ability of older subjects to perform the exercise before and after a fatiguing contraction;(3) in order to test whether rehabilitative interventions might attenuate the ge-related changes in motor unit behavior, we plan to examine the effects of a physical training program on motor unit activity during the isometric exercise. The single motor unit potentials will be measured with a new technique, which utilizes a subcutaneous, branched, bipolar electrode, and a unique waveform discrimination procedure. Preliminary studies have indicated that these procedures are appropriate for the proposed project and that motor unit behavior in older subjects is substantially different from that seen in younger subjects. It is anticipated that these studies will contribute new and useful information on fundamental issues in aging and motor control.